Method for making schmidt corrector lenses

ABSTRACT

A method and system for making Schmidt correctors wherein the surface of a master glass template is ground and polished to an accurate inverse curve of a corrector plate, a thin glass workpiece is placed under vacuum on the master glass template in intimate curve conformed glass-to-glass contact over the entire mating surfaces, the template and workpiece being conjointly rotated while grinding and polishing the exposed free surface of the workpiece to a uniform accurate flatness whereafter the vacuum is released and the workpiece is removed from the template in final form. The present method is directed to inverting the workpiece during polishing and grind, so as to overcome irregularities in the workpiece itself.

United States Patent 1 1 [111 3,889,431 Johnson 1 June 17, 1975 METHOD FOR MAKING SCHMIDT 3,837,124 9/1974 Johnson 51/324 CORRECTOR LENSES 3,837,125 9/1974 Johnson 51/284 [75] lnventor: Thomas J. Johnson, Palos Verdes Estates, Calif.

Celestron Pacific, Torrance, Calif.

Filed: Sept. 23, 1974 Appl. No.: 508,794

Related US. Application Data Continuation-in-part of Ser. No. 393,846, Sept. 24, 1974, Pat. No. 3,837,125.

Assignee:

Primary Examiner-Donald G. Kelly Attorney, Agent, or Firm-David H. Semmes [57] ABSTRACT A method and system for making Schmidt correctors wherein the surface of a master glass template is ground and polished to an accurate inverse curve of a corrector plate, a thin glass workpiece is placed under vacuum on the master glass template in intimate curve conformed glass-to-glass contact over the entire mating surfaces, the template and workpiece being conjointly rotated while grinding and polishing the exposed free surface of the workpiece to a uniform accurate flatness whereafter the vacuum is released and the workpiece is removed from the template in final form. The present method is directed to inverting the workpiece during polishing and grind, so as to overcome irregularities in the workpiece itself.

5 Claims, 11 Drawing Figures METHOD FOR MAKING SCHMIDT CORRECTOR LENSES CROSS-REFERENCES TO RELATED APPLICATIONS:

A continuation-in-part of applicants earlier filed Method and System for Making Schmidt Corrector Lenses" (Serial No. 393,846), issued Sept. 24, l974 or U.S. Pat. No. 3,837,125.

BACKGROUND OF THE INVENTION:

(l) Prior Art:

New Procedure for Making Schmidt Corrector Plates, Applied Optics, Volume I I, No. 7, July. I972. The Vacuum Method of Making Corrector Plates, Sky and Telescope, June, I972. Making Corrector Plates by Schmidts Vacuum Methods, Applied Optics, May, I966, Volume 5, No. 5, pages 713-7l5. US. Pat. No. 3,693,30l. Lemaitre. Study of the Fabrication of Aspherical Surfaces, Sakurai & Shishido, Applied Optics, November, 1963.

There are two techniques well known in the literature for figuring Schmidt corrector plates, the first of which is often referred to as a classical approach and the second approach is referred to as the vacuum deformation techniques, as evidenced, in part, by the aforementioned cited art.

The first approach, the classical approach, involves using a glass blank of high optical quality and of sufficient thickness so that one side can be worked without the glass blank slightly bending or deflecting due to work temperature or pressure during the grinding and polishing phase. The Schmidt curve is ground into the surface by rotating the blank about its center and using grinding laps which favor the areas where more glass is to be removed. It is important in this process that most of the grinding and shaping work be done with the rigid glass blank being rotated about its center on or under a grinding tool that contacts the entire surface, the grinding lap also being rigid, and this tends to keep the glass plate a perfect figure of revolution which is essential. After the shape is roughed in by coarser grits, the grinding lap is cleaned thoroughly and finer grits are applied. This is repeated using progressively finer grits until the plate is ready for polishing. It is during the fine grinding stage that the figure is checked optically. The corrector is set up with the balance of the optical system with which it is to be used and tested with an optical collimater. Null testing techniques and interferometer techniques are commonly known, which allow a worker to read the errors in glass to determine the zones which need to be worked down. Using this classical approach, a high degree of skill and training are required to read the errors and properly interpret them.

The second approach, the vacuum deformation technique, is attributed to Schmidt, the original inventor of the Schmidt corrector plate. This approach consists of using a thin glass blank as a cover for a vacuum-tight chamber. When the vacuum is applied, the thin glass blank bends into the shape of a catinary curve rather than a spherical shape. While in this bent configuration. the top side of the blank is ground and polished spherical. When the vacuum is released and if the processing has been successful, there purportedly would result a perfect Schmidt corrector plate. The vacuum deformation approach implies that a thin plate can be bent sufficiently accurately and will remain sufficiently stable during the grinding procedure to result in :1 usable Schmidt plate. For a visual telescope, the requirements on 21 Schmidt plate are so demanding that the residual errors must be a small fraction of a wave length of light or accuracy approaching a millionth of an inch. If an O-ring is used as suggested in the Sky and Telescope article, small inhomogenouities in the glass will result in non-uniform bending and therefore astigmatism or nonconcentricity about its center. This approach is barely acceptable for use with a camera and would hardly be adequate if the corrector were to he used in a visual Schmidt Cassegrain system for example.

If in using the vacuum deformation approach, the O- ring is dispensed with and a rigid ring used, the ring would have to be perfectly shaped and even then the smallest bit of dust or film at the interface between the glass and ring would still render the resulting plate of poor quality.

In the Applied Optics, November, I963 article. it was suggested that a thin glass blank be bent over a mold having the inverse to the Schmidt curve. The opposite side would be ground and polished flat while in this bent configuration. The article did not specify as to how the glass was to be held in place or as to the accuracy with which the mold should be made. nor how it would be determined that the glass was actually in Conformation with the mold. The article further was devoid of a teaching of the material from which the mold was to be made.

The present invention teaches a method and system which overcomes the drawbacks of the prior art and results in the possibility of producing high quality corrector lenses at a comparatively extremely high rate with a high degree of accuracy.

BRIEF DESCRIPTION OF THE INVENTION:

A method and system for making Schmidt correctors wherein a thick glass master block is figured to the inverse curve of the Schmidt plate, mounted on a spindle of a grinding and polishing machine, the master block having a bore therethrough, grinding and polishing the master block into a master glass template. A thin test piece of glass having shallow grooves therein is placed on the master glass template with the grooves at the interface, and a vacuum is applied through the bore to conform the test piece to the inverse contour curve on the master glass template. The workpiece is drawn and held in intimate curve conformed glass-to-glass contact over the entire mating surfaces under the vacuum and the test piece and template are rotated while grinding the upper surface of the test piece flat. The test piece is removed and tested for accuracy, noting and marking zones having excess glass thereon. The master glass template is then reground and polished to remove re sidual material. The test piece is repetitively replaced on the master glass template with the upper top side of the test plate being reground and polished to flatness and subsequently tested for accuracy of the ground inverse curve on the master glass template which is repet itively reground and polished until the desired degree of accuracy of contour is achieved. The so contoured surface of the master glass template is then grooved and a corrector plate blank is placed on the finalized template in intimate sealing contact therewith under vacuum and the upper surface of the blank is ground to accurate flatness to finalize the corrector. Additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description of an embodiment thereof when taken together with the accompanying drawings in which:

The present invention includes the modification of inverting the workpiece during polishing and grinding, so as to overcome irregularities in the workpiece. Customarily such irregularities prevent intimate Contact at the interface between the workpiece and the master contour block. For example, a thin workpiece having small irregularities on the side in contact with the master glass template is nevertheless ground and polished flat on its top or exposed surface while in best achiev able contact with the master. Thereafter, the workpiece is removed from the master and then reversed contacted with the master is ground, if need be, and polished flat. This modification has extended to thicker workpieces which are impossible to deform to intimate contact with a master contact block.

Also, the workpiece topside may be ground to a positive or negative spherical surface, then polished and tested with deviation from flatness, for the express purpose of shifting the neutral zone of the Schmidt corrector so produced to a smaller or larger radius.

Also, according to the present invention a thick Schmidt corrector lens, having the same oversized diameter, as a master contour block may be produced by figuring flat on one side and having all of the Schmidt curve on the opposite side. Thereafter, the cartwheel pattern, which serves as a channel for the vacuum or compressed air is ground into the lense on the side having the Schmidt corrector curve. The Schmidt corrector lense then serves as a master contour block. Deviations in the curve of the master contour block are noted by the interference fringes between the thick corrector lense being used as a template and the contour block under preparation when they are placed in Contact. The cartwheel pattern is then used with the compressed air to assist in removing the contour block from the thick corrector lense template after the errors in the contour block surface have been noted.

FIG. 1 is a section through a glass master block to be contoured;

FIG. 2 is a schematic elevational view, partly in section, of apparatus for grinding the master glass template;

FIG. 3 is a section of a thin test piece of glass used in practicing the invention;

FIG. 4 is a plan view of the test piece of FIG. 3;

FIG. 5 is a fragmentary sectional view showing application of a test piece to the master glass template for grinding of the upper surface of the test piece;

FIG. 6 is a fragmentary exploded view disclosing the test piece removed from the template after grinding;

FIG. 7 is schematic view for flatness surface grind determination of the test piece;

FIG. 8 is a schematic drawing of the final testing for flatness of the test piece;

FIG. 9 is a sectional view of a Schmidt corrector as made from a master contour block or template having one-half of a desired correction;

FIG. 10 is a schematic fragmentary view of the corrector plate of FIG. 9 placed on a full correction contour block for further processing; and

FIG. 11 is a sectional view of a resulting corrector having one-half of curve on each side.

Referring now in detail to the drawings, a thick glass block 20 of good anneal and stability and having a larger diameter than the desired Schmidt plate is carefully figured to the inverse curve of the Schmidt plate. Special processes for developing this curve are described hereinafter. The glass block which is later to become a master glass template 22 is proved with a through extending bore 24 and mounted on a backing plate generally indicated at 26 which in turn is mounted on the spindle 28 of a grinding and polishing machine generally designated 30. The backing plate spindle 28 is hollow and mounted in a hollow shaft 32. A drive pulley 34 is operatively connected to rotate the spindle assembly. The shaft is mounted for rotation in bearings 36. A rotating vacuum coupler 38 interconnects shaft 32 and the backing plate spindle through a tube 40 of the like having a cutoff valve 42 to a vaccum source or reservoir.

A thin test piece of glass 44 is prepared for use. The surface must be quite good, free from orange peel effect typical of quick polishing and reasonably flat, within about one-half fringe per inch. Such shallow grooves 46 are cut into the test piece in a cartwheel pattern. The test piece for a 8 inch f/Z corrector would be typically I0 inches in diameter and the circular groove would be at about 9 inches diameter and concentric.

The inverse curve of the corrector plate is ground and polished into the master glass template 22 which might be referred to as the contour block. The thin glass test piece 44, as well as the contour block 22, are carefully cleaned and the test piece is placed on the contour block with the grooves at the interface. Vacuum is now applied to the apparatus. The thin test piece will deform to the curve that is on the contour block. Any film, dirt particles, or areas of nonconformation are readily noted by noting the interference fringes at the interface of the test piece and the contour block. A commonly used method to determine whether or not a spherical surface on glass meets its required accuracy is to make a master glass having the opposite curve. The two pieces are placed one on the other, the combination held under a light source (flat and usually monochromatic). The reflections at the interface between the master and the workpiece are observed. If they are very close to the same curve, the light from one of the surfaces will interfere with that from the other and Newtonian rings or interference fringes may be observed. The character of these interference fringes is such that the worker may accurately determine the relative match between the master and the workpiece. If the two glass surfaces exactly match, and if they are perfectly cleaned, then placed in contact with each other, any reflection at the interface will disappear. The two pieces of glass will go into a semi-molecular bond and may result in difficulty separating the two pieces of glass. The bond is of such strength and character in this case that some glass worker will bond a number of small pieces of glass that are all to be parallel and the same thickness to a larger master glass flat. An additional bonding agent may be placed around the edges of the smallerpieces. These pieces hold so rigidly together that they can now be ground and polished as a unit. The pieces are usually separated by reducing the temperature of the master in a refrigerator until the workpieces snap off.

As shown in FIG. 5, the test piece placed on the contour block 22 in the apparatus can have a wax seal 48 applied at the edges. Perfect conformation of the workpiece to the contour block is achieved when all reflection at the interface has disappeared and intimate glassto-glass contact is then achieved.

The top surface of the test piece is then ground flat by rotating the spindle on the machine and using a flat grinding lap driven by the over arm of the machine. It may be polished out as desired. The vacuum is then released, see FIG. 6, and the test piece 44 removed from the contour block. Since the workpiece will tend to bond to the contour block where intimate glass-to-glass contact has been achieved, care must be exercised in separating the pieces. If compressed air is introduced through the spindle, for example, the pieces may separate too quickly as the pressure is slowly increased. The best method of separating the pieces is to place a thin piece of metal at the edges, such as a razor blade, and slowly pry the piece up. As will be seen, the test piece 44 after being so ground and released, will have at least an approximate surface contour as indicated at 50.

The test piece may be set up with a primary mirror and a secondary mirror and tested. The zones where there is excess glass are marked and the contour block is further figured by grinding and polishing or simply polishing, depending on how large the residual errors are at each test. Zones of excess glass on the test corrector are areas where depressions occur on the contour block since it has the inverse curve. After the contour block has been reworked the procedure of placing the workpiece in contact and regrinding its top side flat is repeated over and over until the desired curve on the contour block is achieved.

After the final figure of reverse curve to the required degree of accuracy is achieved, the contour block itself is grooved in the cartwheel pattern of FIG. 4. These grooves are typically 0.010 inch deep and 0.01 to 0.02 wide. In any event, the width of these grooves must be very small compared to the thickness of the desired finished Schmidt corrector plate. If not, there would be some bending of glass into the grooves which would deteriorate the performance of the finished lens. Grooves having a 0.02 width are satisfactory when the finished corrector thickness is about 0.l25 thick or thicker.

It is pointed out here that the procedure of developing the inverse curve on the contour block is even more laborious than producing a single corrector lens by the classical method. After it has been made, however, high quality corrector lenses can be reproduced at a rate of about 1 plate in a 4 hour period for each machine in use. A single operator can easily handle as many as 12 machines after he has developed some skill. In producing Schmidt corrector lenses using the technique described the only critical stages in the procedure are in the cleaning of the workpiece and the contour block so that intimate glass-to-glass contact be achieved over the entire surface. The worker must develope clean working habits so that scratches do not occur on the workpiece during the fine grinding and polishing periods. The worker checks the surface of the workpiece after each progressively finer grit with a spherometer capable of reaching to 0.000] inch. The spherometer base should extend over about two-thirds of the diameter of the workpiece. This is schematically shown in FIG. 7. The spherometer is first checked on a reference flat, then the deviation from flatness of the workpiece is read. During the grinding stage of the corrector on the contour block a spherometer is used to check for flatness of the top side of the workpiece. The plate is ready for final polish when the finest grit has been finished in the grinding stage and flatness to within about 0.0005 inches on the spherometer is achieved. During the polishing stage flatness is checked several times to make sure that the surface remains uniformly flat.

Final test for flatness may be made by placing a small optical flat 54 (FIG. 8) on the surface of the corrector while it remains in contact with the contour block. A typical corrector might be a fringe aspheric, this means that the desired degree of deviation from a flat surface of the finished corrector is 100 fringes or approximately 0.001 inches. When testing the final flat ness deviation a satisfactory corrector figure is indicated if the top surface does not deviate more than 3 or 4 percent total and that this deviation is a smooth spherical section. In this illustration an 8 inch corrector having a l00 fringe aspheric figure should have its top surface flat to within plus or minus about one-half fringe per inch. but this deviation should be constant over the entire surface. The optical flat is placed on the top side of the ground and polished corrector lens while still in contact with the master. Interference fringes appear at the interface between the flat and the plate if they are well cleaned and near the same curve. The shape and number of these fringes tell the worker the shape of the plate being worked. If extending the tech nique beyond the limits set by maximum glass deformation before fracturing occurs, the following is recommended.

From an optical performance point of view the corrector plate need only be thick enough to support its own weight in the instrument and not be so fragile that it must break under normal instrument usage. For this reason the optimum thickess of an 8 inch corrector lens might fall in the range 0.15 to 0.20 inches. It may be desired to fabricate a corrector which requires a curve greater than the glass will safely deform. As an illustration, an 8 inchfll .5 Schmidt camera calls for a corrector having a curve in excess of the amount that 0.l25 glass can be deformed. In this case a portion of the correction is ground onto one side of the glass and then the balance of the correction is ground onto the other, i.e., ground and polished. For this purpose, a contour block 56 (FIG. 9) which has been figured to produce an 8 inch f/2 can be used. Then it is reversed (FIG. 10) and placed in contact with a contour block that has been figured to produce the 8 inchf/LS. The maximum deviation that can be accomplished, then, on a Schmidt corrector is set by approximately half of the curve being applied to each of the two sides.

Subsequent to this mounting as shown in FIG. 10, the upper surface of the piece 56 is again ground flat as delineated hereinbefore, and the resulting corrector 58 is shown in FIG. I] having one-half of the curve on each side. Of course, the material used in the glass block may be fused quartz CerVit or other material which is stable and of good aneal and capable of being polished to the desired inverse curve of the Schmidt corrector lense.

The present method may also be practical when the side of the thin work piece may have small irregularities which make it difficult or impossible to achieve intimate contact at the interface between the work piece and the master contour block. Such work piece is nevertheless ground and polished flat on its top surface while in best achievable contact with the master. Thereafter the work piece is removed from the master and then reverse contacted with the master and the side of the work piece which was previously in contact with the master is ground if need be. and polished fiat.

Also, the work piece may be first contacted to a master contour block having lesser accuracy of figure than that desired to achieve the final Schmidt corrector accuracy. The work piece is then reverse contacted to a master contour block having the desired accuracy of figure. Thereafter the top surface of the work piece is ground and polished fiat while in contact with the contour block.

The method may be practical. also. when the work piece is slightly thicker than will allow it to deform to contact throughout the entire area of the master contour block. Such work piece is nevertheless contacted as on its top side. Thereafter the work piece is removed from the master and its opposite face contacted with the master and the top surface again ground and polished flat.

Also, the work piece top side may be ground to a positive or negative spherical surface and polished and tested with said deviation from flatness. This procedure is performed for the express purpose of shifting the neutral zone of the Schmidt corrector so produced to a smaller or larger radius as may be desired.

When forming the master glass template an intermediate step may involve the producing of a thick Schmidt corrector lens having the same oversize diameter as the master contour block. said thick corrector lens being figured flat on one side and having all of the Schmidt curve on the opposite side. Thereafter the cartwheel pattern which is to serve as vacuum or compressed air grooves is ground into the lense on the side having said Schmidt curve. The thick Schmidt corrector lense now serves as a master contour block. Deviations in the curve of the master contour block being noted by the interferences fringes between the thick corrector lense being used as a template and the contour block under preparation when they are placed in contact. The cartwheel pattern which was ground into the thick Schmidt corrector serves to assist in removing the same from the contour block by the introduction of compressed air after the errors in the contour block surface have been noted.

After the final test for flatness. the workpiece after having been ground and polished on a master template, is thereafter reversely mounted on the same template and the exposed surface is further ground and polished to the degree of flatness desired. This step is used to circumvent any difficulties that might have been encountered in accomplishing. complete and adequate contact with the master template and any glass compressional effects due to the bending of the workpiece over the master template. ln reversely mounting the workpiece to the master template after it has been ground and pol ished the only irregularities to be removed from the exposed surface would be those remaining from imper feet contact when the workpiece was first mounted on the master template.

Also. after the workpiece is initially ground and polished on a template which may have a reduced figure of accuracy from the master template. it is mounted on a master template having the desired accuracy of curve. and the exposed surface ground and polished to the desired degree of flatness.

Manifestly, minor changes in details can be effected without departing from the spirit and scope of the invention as defined in and limited solely by the appended claims.

I claim:

1. A method for forming a master glass template used in making Schmidt correctors. comprising:

A. cutting a thin test piece of glass so as to form shallow grooves in the surface thereof in a cartwheel pattern;

B. mounting said test piece upon a previously ground master glass template contour block having a bore therethrough and being mounted upon a grinding machine backing plate having a hollow spindle through which a vacuum may be drawn, such that the test piece grooves interface with said glass tem plate block and intersect said bore;

C. drawing a vacuum through said grooves and between said block and test piece, so as to deform the test piece to the curve on the contour block as previously ground. vacuum forming an intimate glassto glass contact between block and test piece;

D. grinding the exposed surface of the test piece to a uniform accurate flatness;

E. removing and testing the test piece for accuracy of the desired inverse curve of the block;

F. marking excess glass on said test piece; and

G. further grinding and polishing said testpiece so as to remove residual errors in the contour surface thereof.

2. A method of making Schmidt correctors as in claim 1, wherein said testpiece is removed from the master and reversed contacted with the master such that the side of the testpiece which was previously in contact with the master is around and polished flat.

3. A method of making Schmidt correctors as in claim 1, including initially grinding said testpiece upon a master contour block having a reduced figure of accuracy than that desired to achieve the final Schmidt corrector accuracy. then reverse contacting said testpiece to a master contour block having the desired accuracy of figure and grinding the top surface ofthe testpiece and polishing while in contact with contour block.

4. Method of making Schmidt correctors as in claim 1, including grinding of said testpiece exposed side to a spherical surface and testing deviation from flatness, so as to shift the neutral zone of the Schmidt corrector to a smaller or larger radius as desired.

5. Method of forming Schmidt correctors as in claim 1, including F. producing a thick Schmidt corrector lense, having the same oversized diameter as the master contour block such that said corrector lense is figured flat on one side and has all of the Schmidt curve on its opposite side,

G. subsequently grinding in the cartwheel pattern which is to serve as vacuum or compressed air grooves, such that the Schmidt corrector lense serves as a master contour block.

H. marking deviations in the curve of the master eontour block by the interference fringes between the thick Schmidt corrector lense used as a template and the contour block under preparation; and

I. introducing compressed air into the cartwheel pattern, so as to remove the Schmidt corrector from the contour block. 

1. A method for forming a master glass template used in making Schmidt correctors, comprising: A. cutting a thin test piece of glass so as to form shallow grooves in the surface thereof in a cartwheel pattern; B. mounting said test piece upon a previously ground master glass template contour block having a bore therethrough and being mounted upon a grinding machine backing plate having a hollow spindle through which a vacuum may be drawn, such that the test piece grooves interface with said glass template block and intersect said bore; C. drawing a vacuum through said grooves and between said block and test piece, so as to deform the test piece to the curve on the contour block as previously ground, vacuum forming an intimate glass-to-glass contact between block and test piece; D. grinding the exposed surface of the test piece to a uniform accurate flatness; E. removing and testing the test piece for accuracy of the desired inverse curve of the block; F. marking excess glass on said test piece; and G. further grinding and polishing said testpiece so as to remove residual errors in the contour surface thereof.
 2. A method of making Schmidt correctors as in claim 1, wherein said testpiece is removed from the master and reversed contacted with the master such that the side of the testpiece which was previously in contact with the master is around and polished flat.
 3. A method of making Schmidt correctors as in claim 1, including initially grinding said testpiece upon a master contour block having a reduced figure of accuracy than that desired to achieve the final Schmidt corrector accuracy, then reverse contacting said testpiece to a master contour block having the desired accuracy of figure and grinding the top surface of the testpiece and polishing while in contact with contour block.
 4. Method of making Schmidt correctors as in claim 1, including Grinding of said testpiece exposed side to a spherical surface and testing deviation from flatness, so as to shift the neutral zone of the Schmidt corrector to a smaller or larger radius as desired.
 5. Method of forming Schmidt correctors as in claim 1, including F. producing a thick Schmidt corrector lense, having the same oversized diameter as the master contour block such that said corrector lense is figured flat on one side and has all of the Schmidt curve on its opposite side, G. subsequently grinding in the cartwheel pattern which is to serve as vacuum or compressed air grooves, such that the Schmidt corrector lense serves as a master contour block. H. marking deviations in the curve of the master contour block by the interference fringes between the thick Schmidt corrector lense used as a template and the contour block under preparation; and I. introducing compressed air into the cartwheel pattern, so as to remove the Schmidt corrector from the contour block. 